Solid-state image sensing devices formed as charge transfer devices include both monolithic and hybrid focal plane arrays of the type disclosed in Sequin, et al., Charge Transfer Devices, Academic Press, New York (1975). A plurality of photodetectors is formed on the surface of the semiconductive substrate, each photodetector comprising a diode formed by a diode diffusion in the substrate. Radiation creates a hole-electron pair for each photon incident near the diode, generating a photo-current across the diode junction which is integrated to form a charge packet. Each charge packet is periodically loaded into one bit of a CCD serial output register, the contents of which are serially transferred from the CCD register as a video output signal.
The CCD serial output register is typically constructed by forming a serial succession of plural insulated electrodes overlying the semiconductive substrate, complementary clock signals being applied to alternate ones of the electrodes so that alternate pairs of electrodes define a single charge storing bit of the CCD register, as described in the Sequin publication referenced above. Charge storing bits are formed in the substrate surface beneath alternate ones of the electrodes as potential wells in the electrical surface potential of the substrate. In this manner, the CCD register has a serial train of adjacent successive charge storing bits, so that a single charge packet inserted into one of the charge storing bits of the register is serially transferred down the length of the register at a rate governed by the complementary clock signals. It is apparent that the charge storing capacity of each charge storing bit of the CCD register is limited by the area of the corresponding overlying electrode. If this charge storing capacity is exceeded by injection of an overly large charge packet, a portion of this large charge packet will be lost so that the signal which it represents is severely distorted.
The dynamic range and the signal-to-noise ratio of the video signal generated by the plurality of photodetectors is significantly improved by increasing the integration period during which each photodetector generates a photo-current which is accumulated to form a charge packet. Increasing the integration period must necessarily increase the size of the charge packet so that it may exceed the charge storing capacity of the CCD output register. Therefore, the integration period of the focal plane array of photodetectors is limited by the charge storing capacity of the CCD output register, thus significantly limiting both the dynamic range and the signal-to-noise ratio of the video output signal. This problem is especially severe in staring focal plane arrays in which the focal plane of photodetectors constantly views the same field of view and is not scanned. While the problem of a limited integration period has been at least partially solved by scanning the image across a plurality of columns of photodetectors in a scanning focal plane array, the problem still remains that the integration period of staring focal plane arrays is significantly limited by the charge storing capacity of the CCD output register.
The problem of a limited integration period in image sensors is particularly significant because any image sensor operates with a certain amount of noise, electronic and otherwise, over which the image signal must be superimposed. Furthermore, the contrast of dark and light portions of the image may impose severe requirements upon the dynamic range of the image sensor. Therefore, it has been a goal in the art to provide a CCD image sensor having a long integration period to provide good signal-to-noise ratio and high performance quality under dimly lit or indoor lighting conditions while at the same time having large dynamic range so that under bright conditions a long integration period does not exceed the system capacity. While one obvious way to achieve these goals would be simply to increase the charge storing capacity of each serial bit of the CCD output register, such an increase in capacity would typically require an increase in the electrode area of the CCD register, thus increasing the size of the device, sacrificing one of the chief advantages of forming the image sensor as a miniaturized charge transfer device or CCD.